maint: enforce policy wrt VIR_ERROR and VIR_ERROR0

* cfg.mk (msg_gen_function): Add VIR_ERROR and VIR_ERROR0,
so that sc_libvirt_unmarked_diagnostics will check them, too.

cfg.mk

@@ -358,6 +358,8 @@ msg_gen_function += virXendError
 msg_gen_function += xenapiSessionErrorHandler
 msg_gen_function += xenUnifiedError
 msg_gen_function += xenXMError
+msg_gen_function += VIR_ERROR
+msg_gen_function += VIR_ERROR0
 
 # Uncomment the following and run "make syntax-check" to see diagnostics
 # that are not yet marked for translation, but that need to be rewritten